Polycarbonates are commonly prepared by interfacial process in which phosgene is reacted with biphenols (e.g. bisphenol-A) using a two phase reaction medium of an aqueous solution (e.g. sodium hydroxide) and an organic solvent (e.g. methylene chloride). Optimum reaction conditions for rapid polymerization and high yield of polymer typically involve a pH value of the medium of about 10 to 11. Alkali salts are formed during the course of the reaction as phosgene is consumed. The reaction medium normally contains sufficient water to completely dissolve all salts to form a brine phase. Due to undesired side reactions, a portion of the phosgene is wasted and complete consumption of the biphenol typically requires phosgene usage in excess of 15 percent above stoichiometrically predicted amounts.